-LRB- CNN -RRB- -- A woman in California delivered what may be the nation 's second live-born set of octuplets on Monday morning , surprising doctors who expected seven babies .

The first three to seven days will be critical for the infants , Dr. Mandhir Gupta said .

The six boys and two girls -- ranging in weight from 1 pound 8 ounces to 3 pounds 4 ounces -- were generally doing well in incubators following their Caesarean-section delivery at Kaiser Permanente hospital in Bellflower , California , doctors said .

Three of the babies need breathing assistance , but otherwise the eight do n't appear to have serious problems , doctors said at a news conference Monday evening .

`` It was a truly amazing delivery , '' said Dr. Karen Maples , chief of the hospital 's obstetrics and gynecology department .

Doctors initially believed the mother -- whom they did not identify -- was pregnant with seven fetuses . The woman was 23 weeks pregnant when she was hospitalized seven weeks ago and ordered to bed rest . Watch doctors explain what happened ''

Over a seven-week period , a team of 46 physicians , nurses and other staff prepared for the births . When they started the delivery Monday -- more than nine weeks before the babies would be full term -- they were in for a surprise .

`` After we got to Baby G , we were surprised by the discovery of a Baby H , '' Maples said .

Getting the number correct with ultrasounds before delivery is difficult with so many babies , said Dr. Harold Henry , the hospital 's chief of fetal medicine .

`` It is quite easy to miss a baby when you 're expecting seven , '' Henry said .

The hospital said the woman did n't want her personal information released to the news media , and it would not answer questions about whether she 'd had fertility treatments .

Preliminary research indicates this is the second set of live-born octuplets in the United States , according to the hospital .

Eight babies believed to be the United States ' first set of live octuplets were born in Houston , Texas , in 1998 . One of the infants died days after birth . The seven other siblings recently celebrated their 10th birthday , the Houston Chronicle reported last month .

The first three to seven days will be critical for the California babies , said Dr. Mandhir Gupta , one of the doctors at the news conference . The infants could be in incubators for six to eight weeks and in the hospital for 10 weeks , Gupta said .

The mother is doing `` very well '' after the deliveries , which took about five minutes , Gupta said .

`` She is very excited that she -LSB- has -RSB- all these babies and that the babies are looking good so far , '' Gupta said .

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Babies may be U.S. 's second set of live-born octuplets , California hospital says

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Doctors expected seven babies , did n't know of eighth until delivery

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Octuplets delivered by Caesarean section Monday morning

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Three require breathing assistance ; next seven days will be critical , doctor says